Given a string of digits from 2–9, return all possible letter combinations (phone keypad layout).
Example 1
Input: digits = "23"
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
Explanation: Digit 2 → abc, 3 → def.
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
Explanation: Digit 2 → abc, 3 → def.
Example 2
Input: digits = "2"
Output: ["a","b","c"]
Explanation: Three letters for one digit.
Output: ["a","b","c"]
Explanation: Three letters for one digit.